One or more deformations present within a sheet of a medium to be printed can cause serious reliability problems in a printing system, such as an inkjet printing system, where there is only a small gap between a sheet transport mechanism and an image forming device or printing head of the printing system. If the sheet to be printed touches the image forming device or the printing head as a result of such a deformation, this can lead to print quality degradation and/or to a sheet jam in the machine. To achieve high print quality in an inkjet printing system, the distance between the printing heads and sheet to be printed should be kept small. Because of this small distance (print gap) the print heads are easily touched by the sheets as they pass. Accordingly, even small defects like dog ears, wrinkles, tears etc. can cause a so-called “head touch”, which can degrade print quality, cause nozzle failure, or even sheet jams.
To address these issues, systems have been developed which employ a proofing device capable of identifying sheet deformations and rejecting sheets that contain such deformations. However, there are many sources of defects or errors that may degrade the productivity of a printing system. For example, sheets to be printed supplied to a printing machine may already contain various defects. Also, defects and wear within the machine can cause the sheets to become damaged. Changes in the environmental conditions can lead to deformation of the sheets as they are being processed, and inappropriate settings in a printing system, such as too much ink or a drying temperature that is too high or too low, can also generate problems. Furthermore, such influences or defects can act in combination, so making it very difficult to identify a root cause of a problem.
Drawback of the proofing methods according to the prior art is that they require the user to purchase more sheets than the number will actually be printed, due to sheets being deemed unsuitable for printing. In unfavourable conditions, these methods might reject large quantities of sheets or even the entire sheet stack intended for printing a print job. This results in high costs for an operator, as large numbers of additional sheets are required for completing the print job. For example, the print job prescribes a certain paper type prone to wrinkling. In the prior art the suitability testing devices may reject a large number of sheets of said paper type, because the wrinkles in certain sheets exceed a predetermined threshold value. Since the operator is unable to determine a root cause leading to the wrinkling, he or she is forced to keep supplying sheets until the job is finished. Thereby, the number of sheets input into the printing system may significantly exceed the number of sheets printed, as prescribed by the print job. This leads to additional costs, especially when the rejected or left over sheets cannot be used in a further print job.
US 2004\100016 A describes a media qualification device configured to sort media sheets according to one or more detected media characteristics into one of an unusable media path and at least one usable media path. The media qualification device may further comprise an input/output device configured to select media grade parameters.